The Impatiens (Impatiens walleriana) plant has been the go-to flowering annual for shady locations for decades. All summer, you could rely on its low mounding foliage for producing a long edge of riotous color. Gardeners depended on the beloved plant to skirt homes and tall trees in heavily shaded landscapes.
Our love affair with busy Lizzies (a common name for Impatiens walleriana) gave us inexpensive dependable blooms year after year!
Then early in the twenty-first century, plant professionals noticed a white fungus decimating impatiens in Europe.
Decoded Plants’ Chris Eirschele recently talked with Colleen Warfield, Ph.D., Corporate Plant Pathologist of Ball Horticultural Company, and Ryan Hall, Product Manager of Ball FloraPlants, about the effects of Impatiens Downy Mildew.
Dr. Warfield told Decoded Plants, “We first saw warning signs of downy mildew in Europe in 2003 and 2004. We had been watching this in Europe for nearly seven years and waiting and wondering why it was not here in the United States.”
It was not a matter of If the pathogen was going to arrive in the United States; it was a matter of When.
Now, we see the realization of the plant community’s worst fears. Impatiens walleriana has been difficult for home gardeners to come by and busy Lizzies’ faithful followers are wondering whether they will ever be able to plant their beloved impatiens under tree canopies again.
Downy Mildew Disease Appears in Home Gardens and Landscapes
Impatiens downy mildew is a foliar disease that grows inside the leaves of Impatiens walleriana. Another favorite of gardeners, New Guinea impatiens is a completely different species (Impatiens hawkerii). This pathogen doesn’t hamper the New Guinea impatiens, so it has become an alternative to the impatiens bedding variety.
“Impatiens downy mildew is a very host specific plant pathogen,” stresses Dr. Warfield. “It comes down to genetics.” A fungus-like plant pathogen called Plasmopara obducens causes impatiens downy mildew.
The plant pathogen is most destructive when the environment is ideal.
“Impatiens downy mildew; they like environments that are very moist and wet, and we know the pathogen requires at least 4 hours of free moisture on the leaf.
“You need to have high humidity, have to have free moisture, cool temperatures, particularly when it drops at night in the high 50s low 60s, those are ideal conditions for the disease to progress quickly,” said Warfield.
The air or splash-back from overhead watering in tight plantings carry the spores. Experts say the disease is “latent,” or lying dormant or hidden, until the environmental factors converge again.
Overwintering spores may survive in plant debris; resting spores may survive for years – making it difficult to predict where and when in the landscape impatiens downy mildew will hit gardens. As an example, in the United States in 2012, Plasmopara obducens showed up very early in the Northeast and in New York State and then reappeared in 2013, and 2014, very late in the season.
In 2012, in North Dakota, the downy mildew showed up… and then in the last three years has not returned.
Is it OK to Keep Planting Every Year in the Same Bed?
Decoded Plants wondered: What is the impact of home gardeners repeatedly planting year after year in the same bed?
Warfield explains, “In this particular pathogen, overwintering surviving year to year in a given bed, the evidence so far is not very strong to support that and that’s based on the fact that people have gone back and replanted into beds with a history of the disease and the following year that particular bed is not any more likely to become infected than in a bed that had no history of impatiens in it previously, and in certain locations where it did not show up again the following year.
“So, all of this is observational and kind of anecdotal. In terms of actual scientific control experiments, there are groups of researchers who are working on that trying to understand what role these overwintering spores might play but there is not strong evidence that it’s playing a large role in the current epidemic.”
Home Gardeners Starting with Seeds or Plant Cuttings
In recent years, home gardeners have noticed decreasing numbers of Impatiens walleriana; whether as the common bedding plant, the variety which produces a double-flower form, or in the interspecific hybrids.
If you prefer growing seeds, the Food and Environmental Research Agency, United Kingdom, as well as Warfield confirmed that there is no evidence, to date, that it is a seed-borne pathogen.
The sad news is, “Whether they start their own from seed or buy seed from a reputable source, if they think they will escape it by planting their own seed, downy mildew may still appear…” cautions Warfield.
Think you might escape the downy mildew’s ill effects by taking cuttings at the end of the growing season, from what appear as healthy looking busy Lizzies? Unfortunately, that effort may also prove fruitless.
Though Plasmopara obducens may not have shown itself all summer; and in fall, when you take cuttings; and grow them indoors all winter, as soon as gardeners put transplants outside, if the environment turns perfect for the latent downy mildew, its destructive spores may suddenly become active.
If you want to grow impatiens, start fresh with new seed or purchased transplants – and dispose of 0ld stems and leaves cleared out at the end of the growing season, rather than putting the debris into compost piles.
Identify Powdery Mildew Versus Impatiens Downy Mildew
Home gardeners should not confuse impatiens downy mildew with the more familiar powdery mildew frequently seen on plants like the annual zinnia or the perennial phlox. Powdery mildew shows itself on the tops of a plant’s leaves and turns foliage yellow after the disease appears.
Impatiens downy mildew grows inside the leaves and shows up on the underside of leaves. This downy mildew attacks seedlings, plants tissue, and young plants of only I. walleriana. Unfortunately, you can’t count on visual signs to appear in diseased plants; downy mildew infects the impatiens before the symptoms show up. Early symptoms include:
- White velvety coating appears on the undersides of leaves.
- Foliage turns a light green or yellowish color.
- Growth becomes stunted at an early stage.
- Finally, the infected impatiens will drop their flowers and leaves, and inevitably the plant collapses.
New and Old Plant Ideas for Shade Gardeners
Experiment with new, and old, ideas in a shade garden to replace your beloved Impatiens walleriana. Ryan Hall, Product Manager for Ball FloraPlants, shared some of the new ideas plant breeders and growers are working on which you will see in the coming years.
Hall told Decoded Plants, “Impatiens will always be an important plant shade gardeners want to use. I see it as still important in the market; an idea for small landscapes or in limited spaces, such as in a container garden, versus the lone plant in a long winding planting bed.”
New Guinea impatiens (Impatiens hawkeii) is not a new idea; gardeners originally used them where they wanted that feel of impatiens, but in a sunnier location.
Now home gardeners, as well as professional growers, look to New Guinea impatiens to fill the void made by decimated busy Lizzies. Though New Guinea impatiens have a range of colors, the plant’s more stiffly upright habit and pointy leaves takes getting used to for planting en masse.
Hall shared that plant breeders are working toward interspecific subspecies of Impatiens to produce new plants, like Bounce and Big Bounce. The blooms are bigger than the I. walleriana of old, but the new plant features an improved spreading habit over the species-specific New Guinea impatiens and will soften stone walkways and thrive in shade.
Coleus (Solenostemon scutellarioides) is another beloved plant resurrected to fill in the shade garden. As Hall said, “We’re breeding in coleus, focusing in areas such as landscape application; coleus that can take full sun in locations like Florida, and pushing the flowering later into the season so the plants hold up longer.”
Shade gardeners, who want to use coleus to replace Impatiens walleriana in their planting beds, should look for varieties that thrive in shadier locations.
Coleus has an array of colors in the plants’ foliage rather than in flowers; you may have to experiment with the plant-habit to achieve the “beloved feel” in your shade garden.
Downy Mildew, Impatiens, and Gardeners
Downy mildew has made its way to the United States’ population of impatiens – which means you will need to make a few minor changes. Don’t worry – you still have plenty of options to make the shady areas of you gardens burst with color.